ASHRAE Journal - August 2011 - 42

35

10.375 in.

50

55 60 62 64 65 64 62

30

25 Head (psi)

60 55 5 hp

20 8 in. 50 15 55 1.5 hp 50 55 50 3 hp

10

2 hp 5

0 40 30 NPSHr (ft) 20 10 0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

210

10

20

30

40

50

60

70

80

90

100

110 gpm

120

130

140

150

160

170

180

190

200

210

Figure 3: Pump curve for a liquid refrigerant pump.

Other Considerations
Minimum Flow Protection. While operating, liquid refrigerant pumps always need to move some amount of liquid to avoid forming vapor within the pump due to heat addition from the inefficiency of the pump or motor (if it’s a semi-hermetic configuration). The added heat at low flow conditions can also cause the liquid refrigerant in the pump to boil, leading to cavitation. To prevent this form of cavitation, a bypass (or “minimum flow”) line from the pump discharge back to the pumped recirculator or recirculated liquid return is installed with an orifice (or metering valve) set at a manufacturer’s required minimum flow. During normal operation, the heat gain from the pump power input is small, for the example we used earlier the heat gain would result in a temperature rise of 0.18°F (0.1°C). Presence of EPRs on Pump Discharge Pressure Requirement. Pumping liquid to evaporators equipped with evaporator pressure regulators (EPRs) requires special attention. First, the liquid feed pressure at the evaporator, and thus the pump discharge pressure, must be higher than the EPR set pressure to get liquid into the evaporator. Second, it is important to realize that the pumped liquid refrigerant being supplied to an EPR-fitted evaporator is subcooled as it enters the evaporator. The liquid refrigerant entering the evaporator is essentially at the temperature of the liquid in the pumped recirculator (plus a
42 ASHRAE Journal

slight temperature rise due to heat gain in the pump and in the liquid piping). Since the liquid refrigerant supply temperature is below the saturation temperature corresponding to the EPR pressure setting, the liquid refrigerant entering the evaporator has to absorb heat to sensibly raise the refrigerant temperature until it reaches its saturation temperature corresponding to the operating pressure prior to its boiling. Evaporators operating with liquid refrigerant supply temperatures more than 10°F (5.6°C) below saturation temperature will likely result in poor evaporator performance. Static Head Requirements in Open Loops (Why VFDs Are Not Great For Refrigerant Pumps). Another issue with open loop pumping circuits is that the static head requirement sets the minimum discharge pressure to get flow to the evaporators. This means that before there is any flow to the evaporators, the pressure required to lift the liquid to the roof (or to the elevation of evaporators) must be overcome. If the pressure generated by the pump is not sufficient, the pump will operate at its minimum flow with a column of liquid standing in the riser supplying liquid refrigerant to the roof. In other words, the pump is not capable of delivering flow to the evaporators. This is the main reason that variable frequency drives (VFDs) are generally not suitable applications for refrigerant pumping in liquid overfeed applications. Note that a 50 ft (15.24 m) rise
ashrae.org August 2011



ASHRAE Journal - August 2011

Table of Contents for the Digital Edition of ASHRAE Journal - August 2011

ASHRAE Journal - August 2011
Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
2011–12 Presidential Address: Sustaining ASHRAE Through Leadership
Retrocommissioning Older Buildings
Liquid Refrigerant Pumping in Industrial Refrigeration Systems
Technology Award Case Studies:
Hospital Upgrade: Heat Recovery, Geothermal Save Energy
Cool Weather Savings: Using Hybrid Refrigeration Systems for Chiller Retrofit
Special Section
InfoCenter
Standing Columns
Solar NZEB Project
Emerging Technologies
Special Products
IAQ Applications
Washington Report
People
Products
Classified Advertising
Advertisers Index
ASHRAE Journal - August 2011 - Intro
ASHRAE Journal - August 2011 - ASHRAE Journal - August 2011
ASHRAE Journal - August 2011 - Cover2
ASHRAE Journal - August 2011 - 1
ASHRAE Journal - August 2011 - 2
ASHRAE Journal - August 2011 - Contents
ASHRAE Journal - August 2011 - Commentary
ASHRAE Journal - August 2011 - 5
ASHRAE Journal - August 2011 - Industry News
ASHRAE Journal - August 2011 - 7
ASHRAE Journal - August 2011 - 8
ASHRAE Journal - August 2011 - 9
ASHRAE Journal - August 2011 - Letters
ASHRAE Journal - August 2011 - 11
ASHRAE Journal - August 2011 - 12
ASHRAE Journal - August 2011 - 13
ASHRAE Journal - August 2011 - 14
ASHRAE Journal - August 2011 - 15
ASHRAE Journal - August 2011 - Meetings and Shows
ASHRAE Journal - August 2011 - 17
ASHRAE Journal - August 2011 - 2011–12 Presidential Address: Sustaining ASHRAE Through Leadership
ASHRAE Journal - August 2011 - 19
ASHRAE Journal - August 2011 - 20
ASHRAE Journal - August 2011 - 21
ASHRAE Journal - August 2011 - 22
ASHRAE Journal - August 2011 - 23
ASHRAE Journal - August 2011 - 24
ASHRAE Journal - August 2011 - 25
ASHRAE Journal - August 2011 - Retrocommissioning Older Buildings
ASHRAE Journal - August 2011 - 27
ASHRAE Journal - August 2011 - 28
ASHRAE Journal - August 2011 - 29
ASHRAE Journal - August 2011 - 30
ASHRAE Journal - August 2011 - 31
ASHRAE Journal - August 2011 - 32
ASHRAE Journal - August 2011 - 33
ASHRAE Journal - August 2011 - 34
ASHRAE Journal - August 2011 - 35
ASHRAE Journal - August 2011 - Liquid Refrigerant Pumping in Industrial Refrigeration Systems
ASHRAE Journal - August 2011 - 37
ASHRAE Journal - August 2011 - 38
ASHRAE Journal - August 2011 - 39
ASHRAE Journal - August 2011 - 40
ASHRAE Journal - August 2011 - 41
ASHRAE Journal - August 2011 - 42
ASHRAE Journal - August 2011 - 43
ASHRAE Journal - August 2011 - Hospital Upgrade: Heat Recovery, Geothermal Save Energy
ASHRAE Journal - August 2011 - 45
ASHRAE Journal - August 2011 - 46
ASHRAE Journal - August 2011 - 47
ASHRAE Journal - August 2011 - Cool Weather Savings: Using Hybrid Refrigeration Systems for Chiller Retrofit
ASHRAE Journal - August 2011 - 49
ASHRAE Journal - August 2011 - 50
ASHRAE Journal - August 2011 - 51
ASHRAE Journal - August 2011 - InfoCenter
ASHRAE Journal - August 2011 - 53
ASHRAE Journal - August 2011 - 54
ASHRAE Journal - August 2011 - 55
ASHRAE Journal - August 2011 - 56
ASHRAE Journal - August 2011 - 57
ASHRAE Journal - August 2011 - 58
ASHRAE Journal - August 2011 - 59
ASHRAE Journal - August 2011 - 60
ASHRAE Journal - August 2011 - Solar NZEB Project
ASHRAE Journal - August 2011 - 62
ASHRAE Journal - August 2011 - 63
ASHRAE Journal - August 2011 - 64
ASHRAE Journal - August 2011 - 65
ASHRAE Journal - August 2011 - Emerging Technologies
ASHRAE Journal - August 2011 - 67
ASHRAE Journal - August 2011 - 68
ASHRAE Journal - August 2011 - 69
ASHRAE Journal - August 2011 - 70
ASHRAE Journal - August 2011 - 71
ASHRAE Journal - August 2011 - 72
ASHRAE Journal - August 2011 - 73
ASHRAE Journal - August 2011 - 74
ASHRAE Journal - August 2011 - Special Products
ASHRAE Journal - August 2011 - IAQ Applications
ASHRAE Journal - August 2011 - 77
ASHRAE Journal - August 2011 - 78
ASHRAE Journal - August 2011 - 79
ASHRAE Journal - August 2011 - Washington Report
ASHRAE Journal - August 2011 - 81
ASHRAE Journal - August 2011 - People
ASHRAE Journal - August 2011 - 83
ASHRAE Journal - August 2011 - Products
ASHRAE Journal - August 2011 - 85
ASHRAE Journal - August 2011 - Classified Advertising
ASHRAE Journal - August 2011 - 87
ASHRAE Journal - August 2011 - Advertisers Index
ASHRAE Journal - August 2011 - Cover3
ASHRAE Journal - August 2011 - Cover4
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